An antigen (from the Greek ἀντί anti '[da] against', and γεννάω gennaō 'create / give birth') is a structure to which antibodies and certain lymphocyte receptors can specifically bind (the latter can also cause antibodies against the Antigen are produced) whereby the reaction of an organism to this structure or this substance can change specifically.
By somatic gene rearrangement , lymphocytes can form receptors for almost all possible substances. These substances are called antigens. The corresponding receptors of the lymphocytes are called B-cell receptors or T-cell receptors , depending on the type of lymphocytes . Originally, however, the term was only applied to substances that, after being injected into a foreign organism, led to the formation of antibodies. The specific binding of antibodies and antigen receptors to antigens is an essential part of adaptive immunity against pathogens . Antigens as “infectious substances” can trigger an immune response and thus have an immunogenic effect, but not every antigen is also immunogenic (e.g. haptens are not immunogenic). The location of the antigen that is recognized by the corresponding antibody is called the epitope .
Antigens are mostly proteins , but they can also be carbohydrates , lipids or other substances. They can either be recognized or bound by B-cell receptors, T-cell receptors or antibodies (produced by B cells).
Antigens that are recognized by B-cell receptors or antibodies are located on the surfaces of foreign bodies that have penetrated (e.g. on pollen grains , bacterial surfaces and in the feces of house dust mites ) or cells and have a three-dimensional structure there, which is specific to certain B-cell receptors or antibodies can be recognized. Antigens on cell surfaces are called surface antigens . These are used, among other things, to develop vaccines against pathogens or tumors or are examined before a blood transfusion or organ transplant in order to avoid an immune reaction against foreign blood groups .
Antigens, which are recognized by T-cell receptors, are denatured peptide sequences of approx. Ten amino acids , which are taken up by antigen-presenting cells (APC) and presented on the surface together with MHC molecules .
The body's own structures, including antibodies , can act as antigens if they are mistakenly viewed as foreign ( autoantibodies ). This triggers an autoimmune reaction, which in severe cases can lead to an autoimmune disease .
Certain low-molecular substances that cannot cause an antibody reaction on their own, but can only trigger an immune reaction by binding to a carrier protein, are called haptens . These haptens were important in studying the binding of antibodies to an antigen by serving as chemically defined and changeable test subjects. Accordingly, a (mostly higher molecular weight) substance that enables this reaction on its own is called a full antigen , and a hapten is called a half antigen .
Effect and function
The benefit of antigen recognition by lymphocytes lies in the organism's ability to recognize foreign substances against which it has no hereditary encoded receptors. Lymphocytes that bind to the body's own substances (autoantigens) die; lymphocytes that bind to foreign antigens are able to trigger an adaptive immune response.
T lymphocytes (T cells) only recognize antigens when they are presented on the surfaces of other cells. Antigen presenting cells (APCs) are specialized cells of the immune system that present antigens to T cells. The so-called professional APCs include dendritic cells , macrophages and B cells. They take in substances via various mechanisms such as endocytosis , process them and couple them to MHC molecules. These are then presented on the plasma membrane. A T cell with a suitable T cell receptor (TCR) can then recognize the antigen as foreign and is activated if further costimulative signals are present.
B lymphocytes (B cells), which have bound to an antigen with their B cell receptor (the membrane precursor of the antibody), are activated either directly (TI antigen) or with the help of a T helper cell, depending on the antigen. T helper cells that have bound to an antigen-MHC complex and have recognized the antigen as foreign secrete cytokines that stimulate B cells to produce antibodies. Depending on which cytokines are released in the environment, a class switch takes place in one of the classes (Ig G, Ig E, Ig A). Antibodies are secreted by the plasma cells (activated B cells) , bind specifically to the antigen, thus marking the intruder ( opsonization ) and thus leading to phagocytosis of the foreign body. This task is performed by macrophages, for example, which bind to the constant region of the antibodies with their Fc receptors. By recognizing exogenous antigens, intruders such as bacteria or viruses can be fought against without damaging the body's own cells.
The cells of a stranger are also recognized as foreign, because the structure of the glycoproteins on the cell surfaces is different in every person. Therefore, these human antigens have an adverse effect on the transfer of organic material from one human to another, e.g. B. in blood transfusion or organ transplantation . Here, blood group and tissue compatibility must be observed. The transfer of wrong blood groups leads to clumping of the blood; in the case of transplants, the transferred organ can be rejected or the recipient can be damaged by the transplanted organ ( graft-versus-host disease ).
- Wilhelm Seyffert: Textbook of Genetics. Spectrum Academic Publishing House, Heidelberg / Berlin 2003, ISBN 3-8274-1022-3 .
- Charles A. Janeway Jr. including: immunology. 5th edition. Spectrum Academic Publishing House, Heidelberg / Berlin 2002, ISBN 3-8274-1078-9 .